1. Technical Field
The present invention relates generally to fuel control systems and, more particularly, to a method for triggering the determination of the percent alcohol content of the fuel used in a motor vehicle capable of operating on more than one type of fuel.
2. Discussion
Environmental and energy independence concerns have stimulated the development of alternative transportation fuels, such as alcohol fuels, for use in automobiles. Alcohol fuels include methanol and ethanol. A flexible fueled vehicle capable of operating on gasoline, or alcohol fuel, or any mixture of the two fuels, is therefore in demand. Modifications to the engine are necessary when operating on different fuels because of the different characteristics of each fuel. For example, an engine operating on ethanol or E85 (a blend of 85% ethanol and 15% gasoline) requires approximately 1.4 times the amount of fuel relative to gasoline at stoichiometry due to a lower energy content of the ethanol.
Air/fuel ratio in internal combustion engine design is typically considered to be the ratio of mass flow rate of air to mass flow rate of fuel inducted by an internal combustion engine to achieve conversion of the fuel into completely oxidized products. The chemically correct ratio corresponding to complete oxidation of the products is called stoichiometric. If the air/fuel ratio is less than stoichiometric, an engine is said to be operating rich, i.e., too much fuel is being burned in proportion to the amount of air to achieve perfect combustion. Likewise, if the air/fuel ratio is greater than stoichiometric, an engine is said to be operating lean, i.e., too much air is being burned in proportion to the amount of fuel to achieve perfect combustion. Alcohol fuels have a lower air/fuel ratio than gasoline at stoichiometric, so that the engine must be compensated for in the rich direction as the percentage of alcohol in the fuel increases.
In a returnless fuel system, the only time that the concentration of alcohol in the fuel tank changes is when fuel is added to the fuel tank of the flexible fuel vehicle. Therefore, the alcohol concentration only needs to be learned after fuel has been added to the fuel tank or when an error is discovered in the learned value. Previously, there was no way to detect when a possible alcohol concentration change was about to occur. Without proactive triggers, the alcohol learning relied on the ability of the methodology to react properly to an alcohol concentration change. However, continuously monitoring the potentially changing alcohol concentration interferes with normal vehicle operation. Furthermore, proactive triggers make it less likely that an alcohol concentration change could go undetected.
Therefore, it would be desirable to provide a methodology for limiting alcohol content learning to a short period of time following a fuel addition to reduce the impact that the learning has on normal vehicle operation and to relearn alcohol content if an error is detected.